Modular biological reactor systems have been designed to remove trace organic contaminants and bacteria from recycled water in a low-gravity environment. The use of energy efficient biological treatment allows for the removal of trace organics using only oxygen and nutrients. Three separate reactor systems are currently in use. One reactor removes aromatics, one removes halogenated hydrocarbons and the third removes aliphatics from water streams. A different bacterial consortium is used in each of the three reactors. These reactors have been used to remove ethylene dichloride from a feed stream. The reactors were able to remove over 96% of the ethylene dichloride from a 50 ppm feed stream with a 24-hour retention time. Tests were also performed using kerosene and lubricating oil as contaminants in a reactor feed stream. The immobilized bed reactors removed over 99% of the contaminants from the feed using a 12 hour retention time. The reactor systems are also designed to remove all bacterial cells from the recycled water stream and the reactor effluent line and to recycle unused nutrients back to the reactors